Drop the uuid "v4" feature from the library's dependencies #439

Summary

Cargo.toml enables uuid's v4 feature in the main [dependencies] (line 38, and the wasm target's entry on line 43). The library never calls it. It creates UUIDs only with v5:

• src/query/datalog/matcher.rs:733: a keyword entity becomes Uuid::new_v5(NAMESPACE_OID, keyword)
• src/query/datalog/magic_sets.rs:74: a v5 sentinel entity for magic-sets rewriting

Uuid::new_v4 appears only in #[cfg(test)] modules, tests/, benches/, examples/ and three rustdoc examples (src/graph/types.rs:246,255, src/graph/storage.rs:79). The dev-dependency entry (Cargo.toml:71) already enables v4 for all of those.

Ask

• Remove "v4" from the uuid features in [dependencies] and in the wasm target entry. Keep it in [dev-dependencies].
• Check that cargo build, cargo test (including doctests), cargo build --target wasm32-unknown-unknown --features browser and the binary-size check all still pass. Note the size change, if any.
• If any wasm or browser build still needs getrandom/js without v4, decide what that dependency is for and drop it too if nothing uses it.

Why v3.0.0

minigraf does not re-export uuid, but a downstream crate could be relying on Cargo feature unification to get Uuid::new_v4 through minigraf's dependency. Dropping the feature could break that build, so it goes in the major release.

Philosophy check

Aligned with "self-contained": fewer enabled features, fewer transitive dependencies (v4 pulls in getrandom).